Specifically modified decadeoxyribonucleotide helices will be synthesized by solution or solid phase phosphotriester techniques. The modifications include: (1) an alkylated phosphate group (ethyl phosphotriester), (2) O6-methyldeoxyguanosine, (3) 3-methyldeoxyuridine, (4) deoxyuridine, (5) an apurinic site, and (6) a cyclobutane-type thymine photodimer. Similar modifications are found in cellular nucleic acids exposed to environmental mutagenic and carcinogenic alkylating agents or to ionizing radiation. The modifications will be located in the central dimer region of he helix. The helices thus serve as models for DNA which has been altered by reaction with alkylating agents or ionizing radiation. The stabilities and thermodynamic properties of the modified helices will be studied by ultraviolet spectroscopic techniqus, while the mode of base stacking will be observed by circular dichroism (CD) spectroscopic techniques. Both proton and phosphorous nuclear magnetic resonance (NMR) spectroscopy will be used to study the conformations and dynamics of the modified helices. Of particular interest will be the study of the perturbational effects of base and phosphate alkylation on the local conformation at the site of the modification. The modified helices will be tested for their ability to serve as substrates for enzymes involved in DNA replication and repair. These enzymatic activities include DNA polymerase, DNA ligase, DNA N-glycosylase, and DNA apurinic endonuclease. The results of the studies on helix conformation and dynamics will be correlated with the results of the enzyme studies.